[Infra] Flip remaining CI jobs to Python 3.12

Stragglers from the 2026-04-21 Python 3.12 standardization:
- .github/workflows/check_duplicate_issues.yml (was 3.11)
- .github/workflows/llm-translation-testing.yml (was 3.11)
- .github/workflows/scan_duplicate_issues.yml (was 3.13)
- .circleci proxy_build_from_pip_tests (was 3.13)

The only intentional non-3.12 CI job is installing_litellm_on_python_3_13,
which exists as an explicit "latest supported Python" smoke matrix.

.github/workflows/check_duplicate_issues.yml

@@ -39,7 +39,7 @@ jobs:
         if: github.event.action == 'opened'
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
         with:
-          python-version: "3.11"
+          python-version: "3.12"
 
       - name: Auto-close if high-confidence duplicate
         if: github.event.action == 'opened'

.github/workflows/llm-translation-testing.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
         with:
-          python-version: "3.11"
+          python-version: "3.12"
 
       - name: Set up uv
         uses: astral-sh/setup-uv@37802adc94f370d6bfd71619e3f0bf239e1f3b78 # v7

.github/workflows/scan_duplicate_issues.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
         with:
-          python-version: "3.13"
+          python-version: "3.12"
 
       - name: Scan for duplicate issues
         env: